Rechargeable batteries with high energy density, high capacity and long cycle life are highly desirable. Two types of alkaline rechargeable batteries commonly used are the Ni—Cd (nickel cadmium) type and the Ni—MH (nickel metal hydride) type. In both types of batteries the positive electrodes are made with an active nickel hydroxide material while the negative electrodes are different.
Ni—MH cells operate by a different mechanism than Ni—Cd cells. Ni—MH cells utilize a negative electrode that is capable of reversible electrochemical storage of hydrogen, hence the term hydrogen storage battery. The negative and positive electrodes are spaced apart in an alkaline electrolyte. Upon application of an electrical potential across a Ni—MH cell, the active material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, as shown in equation 1.M+H2O+e−⇄M—H+OH−
The negative electrode half-cell reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron through the conduction network into the battery terminal.
The reactions that take place at the positive electrode of the Ni—MH cell are shown in equation 2.Ni(OH)2+OH−⇄NiOOH+H2O+e−
The use of nickel hydroxide as a positive active material for Ni—MH batteries is generally known. See for example, U.S. Pat. No. 5,523,182, issued Jun. 4, 1996 to Ovshinsky et al., entitled “Enhanced Nickel Hydroxide Positive Electrode Materials For Alkaline Rechargeable Electrochemical Cells”, the disclosure of which is herein incorporated by reference. In U.S. Pat. No. 5,523,182, Ovshinsky et al. describes a positive electrode material comprising particles of nickel hydroxide positive electrode material and a precursor coating of a substantially continuous, uniform encapsulant layer on the particles to increase conductivity and resistance to corrosion products. The encapsulant layer may be formed from a material which, upon oxidation during processing or during charging of the electrode, is convertible to a highly conductive form, and which, upon subsequent discharge of the electrode, does not revert to its previous form. The electrochemically active hydroxide may include at least nickel hydroxide and the encapsulant layer preferably includes cobalt hydroxide or cobalt oxyhydroxide.
Two primary forms of positive electrodes exist at present and include sintered and pasted type electrodes. Sintered electrodes are produced by depositing the active material in the interstices of a porous metal matrix followed by sintering the metal. Pasted electrodes are made with nickel hydroxide particles in contact with a conductive network or substrate, most commonly foam nickel or perforated stainless steel coated with nickel. Several variants of these electrodes exist and include plastic-bonded nickel electrodes, which may utilize graphite as a micro-conductor, and pasted nickel fiber electrodes, which utilize nickel hydroxide particles loaded onto a high porosity, conductive nickel fiber or nickel foam. The current trend has been away from using sintered electrodes in favor of pasted electrodes because of cost and because pasted electrodes can provide significantly higher loading.
Several processes for making positive electrodes are also generally known, see for example U.S. Pat. No. 5,344,728 issued to Ovshinsky et al., the disclosure of which is herein incorporated by reference, where electrodes having a capacity in excess of 560 mAh/cc are reported. The particular process used for making electrodes can have a significant impact on the electrode's performance. For example, conventional sintered electrodes may now be obtained with an energy density of 480-500 mAh/cc. Sintered positive substrates are constructed by applying a nickel powder slurry to a nickel-plated, steel base followed by sintering at high temperature. This process causes the individual particles of nickel to weld at their points of contact, resulting in a porous material that is approximately 80% open volume and 20% solid metal. The sintered material is then impregnated with active material by soaking it in an acidic solution of nickel nitrate, followed by conversion to nickel hydroxide in a reaction with alkali metal hydroxide. After impregnation, the material is subjected to electrochemical formation. Pasted electrodes may be made by mixing various powders, such as nickel hydroxide particles, binders and other additives into a slurry and applying the mixture to a conductive grid.
Production methods for making nickel hydroxide powder are generally known and such powder may be made using a precipitation reaction, such as the one described in U.S. Pat. No. 5,348,822, issued to Ovshinsky et al., the disclosure of which is herein incorporated by reference. In U.S. Pat. No. 5,348,822, Ovshinsky et al describes producing nickel hydroxide material by combining a nickel salt with a hydroxide to precipitate nickel hydroxide. Like electrode formation, the method for making the nickel active material can have a significant impact on properties and performance of the electrode.
Nickel hydroxide material should have high capacity and long cycle life. Excellent results have been found by forming nickel hydroxide with an apparent density of 1.4-1.7 g/cm3, a tap density of about 1.8-2.3 g/cm3, and an average size range of about 5-50 μm. Excellent results have also been found by making an active, nickel hydroxide with a high packing density and a narrow size distribution, such as may be provided with substantially spherical particles having an average particle size of about 10 μ and a tap density of about 2.2 g/cc. Paste made with this kind of active material has good fluidity and uniformity, making it possible to fabricate high capacity, uniformly loaded electrodes. The use of this kind of active material also improves utilization and discharge capacity. However, if process conditions are not carefully controlled, the resulting precipitate may be irregular in shape and have a low tap density. Electrodes formed with low-density nickel hydroxide will lack high capacity and high energy density. Improper process conditions can also produce a powder that is too fine. A very fine powder will increase adsorption of water at the surface of the particles, thereby requiring longer filtering times. Further, if process conditions are not properly controlled, precipitated particles may be formed with an excessively wide particle size distribution (ranging from 1 to hundreds of microns). Nickel hydroxide made with an excessively wide particle size distribution may require additional processing, such as pulverization, to render it useful. For these reasons and others, active powder having a low density, irregular shape and/or poor size distribution is undesirable for use in a high capacity nickel metal hydride battery.
To produce high density, substantially spherical nickel hydroxide powder, carefully controlled process conditions are used to seed and gradually grow nickel hydroxide particles. Although process conditions can vary, generally the process involves combining a nickel salt with an ammonium ion to form a nickel-ammonium complex. The nickel-ammonium complex is then broken down, typically with caustic, to gradually precipitate nickel hydroxide. However, this reaction rate is difficult to control, so methods have been introduced to separate certain steps in the production process. For example, U.S. Pat. No. 5,498,403, entitled “Method for Preparing High Density Nickel Hydroxide Used for Alkali Rechargeable Batteries”, issued to Shin on Mar. 12, 1996, the disclosure of which is herein incorporated by reference, discloses a method for preparing nickel hydroxide from a nickel sulfate solution using a separate or isolated amine reactor. Nickel sulfate is mixed with ammonium hydroxide in the isolated amine reactor to form a nickel ammonium complex. The nickel ammonium complex is removed from the reactor and sent to a second mixing vessel or reactor where it is combined with a solution of sodium hydroxide to obtain nickel hydroxide. The nickel hydroxide particles may then be formed into a pasted electrode with suitable binders, additives, conductive powders, etc. The electrode is then combined with a negative electrode, separator and a suitable electrolyte to form a hydrogen storage battery.
One useful form of hydrogen storage battery is the sealed type. Sealed batteries typically use a small amount of liquid and operate in what is called a starved condition. These types of batteries are particularly advantageous since they are maintenance free. However, sealed hydrogen storage batteries are vulnerable to degradation during cycling, particularly, during overcharging and overdischarging conditions. During overcharge the positive electrode produces oxygen and then recombines at the negative electrode with hydrogen. This localized heating in turn lowers the oxygen evolution potential at the surface of the positive electrode and thereby causes excess gas evolution to occur during overcharge, primarily hydrogen gas. The end result is a build-up and venting of hydrogen resulting from gas being generated at a rate faster than can be recombined within the battery. As a consequence, the venting of hydrogen systematically reduces battery cycle life through oxidation of the negative active material and active material disintegration, loss of electrolyte as well as cell capacity, increased cell impedance due to separator dry-out, and effects the balance between the overcharge and over-discharge reservoirs.
To reduce the potential for oxidation of the negative active material and minimize gas evolution, current practice is to make hydrogen storage batteries that are positive limited, e.g. have a positive electrode capacity, which is smaller than negative electrode capacity. Excess negative capacity prevents the negative electrode from becoming fully charged and ideally permits oxygen produced at the positive electrode to easily recombine at the surface of the negative electrode according to the following reactions:OH−→¼O2+½H2O+e− (at the positive electrode)MH+¼O2→M+½H2O (at the positive electrode)
However, positive limiting a battery alone does not prevent premature failure due to complications from over charging or overdischarging . Other mechanisms exist that can lead to premature failure of the battery.
In a pasted type nickel electrode for an alkaline storage battery, the conductive substrate may be made by forming a nickel-plating over a base urethane foam having a high degree of porosity. The urethane is later burned off in an annealing step leaving a nickel skeleton frame of pores. These pores can be several hundred millimeters across and can be filled with a large number of active material particles in each pore of the foam substrate. The greater the pore size the greater the number of active material particles per pore. However, pore size can also affect the distance between particles, and the distance between particles and the conductive substrate. An unoptimized pore size can lead to higher resistivity of the electrode thereby reducing the utilization of the active materials.
In order to reduce electrode resistivity various additives can be added to the active material powder, such as metal cobalt or cobalt compounds. Subsequent electrical formation of the battery causes the above-mentioned metal cobalt and cobalt compounds to be oxidized to β-CoOOH by charging, thus increasing the conductivity of the electrode and improving the utilization of the active materials. These additives, comprising as much as 5-15%, create a conductive network between adjacent active material particles and the foam substrate.
However, even when the correct addition of metal cobalt or cobalt compounds as conductive agents is provided in the positive paste, there can still be problems with the pasted positive electrode when used in an alkaline storage battery. For example, the efficiency of the positive active material in the electrode can be affected during charging under high temperature conditions. During battery formation the cobalt additives dissolve in the alkaline electrolyte solution and re-deposit as cobalt hydroxide on the surface of the active material. However, when the cobalt hydroxide deposits, it does not happen uniformly, causing segregation on the surface of the electrode and only a small part of the cobalt hydroxide diffuses in the pore, thereby reducing the conductivity of the electrode. In addition, charging the positive at elevated temperatures also decreases the oxygen over-voltage potential in the positive electrode. This decreased in the oxygen over-voltage potential changes the point where the side reaction at which oxygen evolution occurs and as a result decreases the charge efficiency characteristics of the active material.
In recent years, much work has been devoted to the positive pasted electrode in an effort to try to reduce the amount of cobalt additives in the positive paste thereby reducing the cost of the battery. However, reducing these conductive additives has also led to poor cycle life, higher internal pressure and loss of capacity.
As a result of the forgoing, there exists a need for an alkaline battery that reduces the amount of cobalt additives in an alkaline storage while maintaining comparable cycle life, cell performance and charge balance.